The present inventive method utilizes a digital imagesetting system for producing a printing plate for a rotary printing press; especially an offset printing cylinder which can be written on digitally and also erased again.
The printing unit may be a customary offset printing press. In such presses three cylinders, the printing-plate carrier cylinder, the blanket cylinders and the impression cylinders, collectively the printing cylinders, cooperate to create a printed image. The data to be printed, i.e. image information, has been transferred on a printing-plate carrier. Using a principle akin to immiscibility of oil and water, the plate is first dampened with water to prevent non-image areas from absorbing oil based inks. The plate is then inked, and an ink impression is transferred from the printing plate to a blanket cylinder. A blanket made of synthetic and/or natural rubbers is secured to the surface of the blanket cylinder and absorbs the ink. A web of paper or other print material is run between the blanket and impression cylinders. As the blanket cylinder revolves, the ink is deposited on the moving web and the image is printed.
The digital imagesetting unit operates on the external drum principle in known systems from the applicant, in which a laser beam uses the stocks of digital data to define the image information on the surface of the printing cylinder with the printing cylinder rotating rapidly. U.S. Pat. No. 6,580,524 from the applicant, which is hereby incorporated by reference in its entirety, describes a method of controlling the creation of an image on a printing-plate carrier in a printing press. In that document, the applicant discloses that the image signals that create the printing plate, i.e. set the image on the printing plate, are retrieved from a memory of a computer and supplied to a device for creating the image.
A raster image processor (RIP) sets up the scanned image via a bitmap memory in the known system, because the image may be composed of a series of dots and lines of dots. Thus, the device for setting the image, such as a laser, may interpret the digital image data correctly from a PostScript file and giving each image point of the subsequent overall form its identity (tonal value) and its position.
The conversion of a stock of digital image data into half-tone values is, for example, done as follows: in order to be able to convert 256 grey stages into half-tone values, the laser must be capable of subdividing the half-tone cell (the half-tone dot) at the selected halftone fineness into 16×16 addressable half-tone elements. In the event that a full tone were exposed, all 256 half-tone elements would be blackened, and in the event of a half-tone value of, for example, 15%, about 38 half-tone elements would likewise be blackened. Each exposed half-tone dot would in this case be assembled from a bitmap of 256 half-tone elements. The bitmap therefore contains the control information for the laser which, from this, exposes a half-tone dot comprising a corresponding number of pixels.
Printing cylinders provide constant print quality only when they are correctly set-up, for the printing plate cylinder that means a correct transfer pressure. A set-up where the transfer pressure is too low yields non-uniform transport of ink or damping solution, because of the tolerances of circular running and cylindricity. A setting where the transfer pressure is too high prematurely wears the cylinder surface, because of internal friction and pressure overload.
For the purpose of standardization, that is to say quality assurance in offset printing, a suitable electronic concept is currently used, which comprises the control desk technology, as it is known, which is understood to mean the automatic management and monitoring of many sequences during printing. Microprocessors and extremely small computers process a large number of data for this purpose, their results ultimately being passed on to the respective control and regulating elements via an appropriate data bus. Professional scanners (densitometers) currently offer many and various possibilities for correcting the tonal and color values. The (half-)tone value in the print in this case indicates the percentage area ratio of half-tone dots and paper white.
Each change to a half-tone dot in the print has its effect in the ratio between covered and uncovered area. As is known, as a result of the transfer process in offset printing, the half-tone dots become larger in most cases, and in this case one speaks of a tonal value gain in the print. If the half-tone dots of only one color become greater than desired, then the result is a different hue. Of course, this has an effect in the overprint. The most important precondition for agreement between half-tone motifs in the initial print and in the edition print is, however, agreement between the tonal value gain of all the colors and printing points.
The tonal value gain is given by the difference between the known half-tone value on the plate which, as described above, itself results from the RIP as a bitmap together with the tonal value change of the exposure, and from the half-tone value in the print, measured by means of a densitometer, for example. The tonal value gain as a deviation of the half-tone value in the print from the half-tone value of the plate or based on the input data can be represented in a printing characteristic curve that can be used directly for the reproduction.
To determine the printing characteristic curve, stepped half-tone wedges with at least three, or preferably five or more half-tone steps and a full-tone area are printed. The color density in the full-tone and in the half-tone steps is then usually measured with the densitometer and used to determine the half-tone values. If the values obtained in this way are plotted in a graph against the corresponding desired half-tone values, the result is the appropriate printing or transfer characteristic curve. For different combinations of printing ink, paper, printing plate and so on, different transfer characteristic curves are of course used (see U.S. Pat. No. 6,580,524).
A further problem for quality assurance results from the requirement for higher and higher productivity, for example as a result of attempts to produce the most lightweight and cost-effective cylinders. In particular channel-less printing, as it is known, in particular the sleeve technique, which is distinguished by a rubber blanket fitted in a seam-free manner to a sleeve and a printing plate laser-welded to form a cylindrical shape, permits a reduced stiffness, because of the reduced excitation to oscillation owing to the lack of cylinder channels. The length/thickness ratio of the printing cylinder and its relative stiffness with regard to deflection therefore become less and less favourable. The consequence of this is that during printing operation the shape and position of the printing cylinders in relation to one another may change in an undesirable way, that is to say the printing cylinders deflect.
The positional change changes the printing pressure, specifically the transfer pressure of the printing plate interacting in the printing unit. This pressure becomes non-uniform when viewed over the cylinder width. This printing pressure is generally determined in numerical values by measuring what is known as the imprint width, that is to say the width of the zone which defines the contact area of the cylinders when they are set against each other, that is to say moved so as to exert pressure. This measurement is particularly simple in offset printing, since here one cylinder of a pair of cylinders always has a compressible (soft) surface.
The printing or transfer characteristic curve, that is to say the tonal value gain, then depends directly on this imprint width, an increased imprint width meaning an increased tonal value gain and vice versa. The effect described therefore leads to a printing characteristic curve which changes in an undesired manner as viewed over the cylinder width.
In order to stabilize these printing characteristic curve values that vary over the cylinder width, that is to say to compensate for the difference between the desired and actual characteristic curves, attempts have hitherto been made to avoid this positional change of a printing cylinder, caused by deflection, by means of mechanical measures or measures connected with mechanical construction. Known countermeasures to deflection nowadays are, firstly, cylinder surfaces of crowned (convex) design and, secondly, the use of extremely rigid materials, but also a design construction of a cylinder group which permits maximum mutual support of the cylinders, so that the pressure which leads to the positional change is somewhat reduced. However, these measures are currently viewed as being too complicated and too expensive.